1. Field of the Invention
The invention relates to a fill level measuring device and method for containers filled with fluid, preferably for tanks of tank installations, wherein a fill level height is determined from a sound transit time of ultrasound pulses, which are emitted by first sound pulse transmitters along a main measuring path extending from the bottom region of the container up to the surface of the fluid and are reflected from the surface of the fluid to first sound pulse receivers.
2. Description of the Related Art
Such a fill level measuring device which is known from Published European Application No. 0 106 677 A3, corresponding to U.S. Pat. No. 4,748,846, operates according to the principle of echo depth sounding systems. In that device, path lengths are derived from the transit time of sound pulses. This transit time or transit time difference is obtained in each case from the measured time difference elapsing between the transmission of an ultrasound pulse until the reception of the ultrasound pulse reflected by the surface of the fluid. The sound pulse transmitters and sound pulse receivers are preferably constructed as transmit-receive test heads, which are suitable for both transmitting and receiving ultrasound pulses. The sound velocity in a fluid volume is dependent on the density. In order to obtain an average, density-corrected sound velocity in the fluid volume, reference reflectors that are distributed over the fill level height at a defined spacing from one another and from the sound pulse transmitters and receivers are provided in the known fill level measuring device. The ultrasound pulses reflected at the reference reflectors produce transit time differences, which are used together with the associated known sound travel distances to ascertain an average sound velocity that takes the density in the fluid volume into account. Additionally, a temperature sensor is disposed in the lower region of the container and with it, the temperature of the fluid at some point can be ascertained. A calculation of an average temperature of the fluid volume is also provided. The average temperature is needed in order to be capable of calculating the density and thus the container contents once the fill level height is ascertained. In order to calculate the average temperature, known linear characteristic curves .DELTA.v/.DELTA.T are the point of departure. They indicate the variation in the sound velocity as a function of the variation in temperature. The known quotient can be made equivalent to a quotient ascertained by measurement, in which a difference v.sub.L -v.sub.Ref is located in the numerator and a difference T.sub.AV -T.sub.Ref is located in the denominator.
Reference symbol v.sub.L stands for an average sound velocity, v.sub.Ref stands for a reference sound velocity, T.sub.AV stands for the average temperature, and T.sub.Ref stands for a reference temperature. The expense and complication in terms of apparatus and electronics relative to the accuracy is relatively high in that known fill level measuring device. Moreover, a number of control measurements must be carried out, because scattered beams occur which have to be eliminated in the measurements as a result of the relatively high number of reference reflectors that are distributed over the fill level height.